Detection and quantification of asynchronies between inspiratory effort and ventilatory assist during mechanical ventilation is complicated. A first relevant information element concerns patient's neural inspiratory effort. Most methods of determining inspiratory effort use the onset of inspiratory pressure, flow, and/or volume or any related integral or derivative thereof to assess the start and end of inspiration. Due to many factors related to respiratory muscle weakness and impaired respiratory mechanics, there are limitations as to the level of disability where pneumatic measurements are of value. Intrathoracic measurement of inspiratory pressures is another approach to determine the start and end of an inspiratory effort. This approach is limited by (a) the use of expiratory muscles, falsely indicating a negative pressure deflection although neural inspiration has not yet commenced as well as (b) application of ventilatory assist that causes the nadir of the negative pressure deflection to occur more and more prematurely as ventilatory assist increases.
Measuring the electrical activity of inspiratory (or related to inspiration) muscles offers an approach that is more directly related to neural activity of respiratory muscles. There are, however, limitations as to how one can reliably obtain these electrical signals. Electrodes placed on the surface of the thorax or neck region may record inspiratory muscle electrical activity, but may also record activity related to posture and active expiration. Signals obtained in the esophagus, at the level of the diaphragm hiatus, reflect diaphragm electrical activity (EAdi), but may include crosstalk from the esophagus itself, its lower sphincter, and the heart.
A second information element relevant to determining patient ventilator asynchrony is the start and termination of the ventilatory assist. Obtaining this information is relatively easy since (a) the state of the mechanical ventilator can be acquired directly from the machine, or (b) the onset of pressure deflection can be detected by measuring pressure/flow/volume signals from the ventilator circuit.
In general, the patient-ventilator asynchrony is related to delays between the onset of neural inspiratory effort and the onset of ventilatory assist as well as between the end of the neural inspiratory effort and the termination of the ventilatory assist. Thus, the asynchrony can relate to (1) ventilatory assist starting before neural inspiratory effort (early triggering) and (2) ventilatory assist starting after neural inspiratory effort (late triggering). Also, the asynchrony can relate to (3) ventilatory assist terminating before neural inspiratory effort (early off-cycling) and (4) ventilatory assist terminating after neural inspiratory effort (late off-cycling). In the extreme, there could be (5) a neural inspiratory effort without any delivery of mechanical ventilatory assist (wasted inspiratory effort) or (6) a full cycle of ventilatory assist delivered in the absence of neural inspiratory effort (auto-triggering). There could also be several cycles of ventilatory assist during a single cycle of neural inspiratory effort or vice versa. Currently there is no efficient method for determining and quantifying all of these situations.
Therefore, there is a need for a standardized and non-biased technique for automatically determining and quantifying asynchronies between inspiratory effort and ventilatory assist during mechanical ventilation. Reliable information can then be used to correct errors in the ventilator settings or indicate need for change of ventilator mode.